The electroless plating of various metals, such as copper and nickel onto a substrate is well-known in the prior art. For instance, an electroless or autocatalytic copper plating bath usually contains a cupric salt, a reducing agent for the cupric salt, a chelating or complexing agent, and a pH adjuster. In addition, if the surface being plated is not already catalytic for the deposition of the desired metal, a suitable catalyst is deposited on the substrate prior to contact with the plating bath. Among the more widely employed procedures for catalyzing a substrate is the use of stannous chloride sensitizing solution and a palladium chloride activator to form a layer of metallic palladium particles.
Although, the technology relative to electroless copper plating is continually being improved, there still remains room for additional improvement. Certain problems are especially pronounced when preparing articles of very high quality, such as those to be employed in printed circuit applications, e.g., printed circuit boards which contain high-density circuitry and large numbers of holes such as through-holes and blind holes. The problems encountered include the formation of voids on the surface and in the coatings located in the holes. This, in turn, can cause unreliable electrical connections. Moreover, even if the electrical connections initially are adequate, the presence of voids tends to cause the coatings to crack during use of the circuits. During operation, integrated circuit boards tend to expand and contract somewhat. Any discontinuities in the coating represent a prime site for cracking due to the mechanical stress from such expansion and contraction.
Accordingly, it would be advantageous and desirable to reduce the formation of voids.